To Determine the Concenteration of a Limewater Solution

To Determine the Concentration of a Limewater Solution

The aim of this investigation is to determine the concentration of a limewater solution using hydrochloric acid with a known concentration of 2.00mol dm-3.

In this investigation, I will be using titration to determine the concentration of the limewater solution. “Titration is a way of measuring quantities of reactants, and can be very useful in determining an unknown concentration.” But before I can undergo the titration, I will need to dilute the hydrochloric acid. This is because the limewater has an approximate concentration of 1g dm-3, whereas the hydrochloric acid has a concentration of 2.00mol dm-3. This is important because if the hydrochloric acid has a higher concentration than the limewater, then not all of the solution will neutralise fully, which will not give us accurate or reliable results.

Before the titration, I will also have to work out which indicator should be used in this experiment. The indicator is essential as it shows the end-point where the entire base has reacted to give a neutral solution. As both hydrochloric acid and calcium hydroxide are strong bases , I have a choice of using either methyl orange or phenolphthalein as my indicators. I would be using phenolphthalein as my indicator, because the phenolphthalein will change colour just before the equivalence point and therefore, too much acid will not be added to the solution. This means it will be much more accurate than something like Methyl Orange, which would change after the equivalence point and after too acid much has been added.

To carry out the titration I also need to know the balanced chemical equation for the reaction.i

Balanced Equation for the Reaction

Ca(OH)2 (aq) + 2HCl (aq) CaCl2 (aq) + 2H2O (l)

1 mole of calcium hydroxide reacts with 2 moles of hydrochloric acid for a neutral reaction.

I need to work out the more diluted concentration of hydrochloric acid so the reaction is neutralised completely.

Concentration of Limewater

Approx.Conc = 1g dm-3

Mr (Ca(OH)2) =40 + 2(16 + 1) = 74

Moles = Mass ÷ Mr

Moles = 1/74

Approx.Conc = 1/74 mol dm-3 or 0.0135 mol dm-3

Moles of Limewater

No. of moles of Ca(OH)2 used in 250cm3 of limewater = concentration × volumes in dm3

= 0.0135 × 0.25

Moles = 0.003375 approx.

Moles of Hydrochloric Acid

Ratio of Ca(OH)2 : HCl = 1 : 2

No. of moles of HCl needed for neutralisation = 0.003375 × 2

= 0.00675 approx.

Concentration of Hydrochloric acid

So we can see from the balanced equation that twice as much acid is used than alkali, I need to use acid with twice the molar to that of alkali to result in equal volumes being used.

Concentration of HCl required = 0.0135 × 2

= 0.027 mols dm-3

So we need to get ...

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= 0.0135 × 0.25

Moles = 0.003375 approx.

Moles of Hydrochloric Acid

Ratio of Ca(OH)2 : HCl = 1 : 2

No. of moles of HCl needed for neutralisation = 0.003375 × 2

= 0.00675 approx.

Concentration of Hydrochloric acid

So we can see from the balanced equation that twice as much acid is used than alkali, I need to use acid with twice the molar to that of alkali to result in equal volumes being used.

Concentration of HCl required = 0.0135 × 2

= 0.027 mols dm-3

So we need to get the concentration of acid down close to 0.027 mols dm-3. So 250cm3 of 0.027 mols dm-3 HCl will have 0.00675 moles (0.027 × 0.25), which means that this is the correct concentration to dilute the HCl down to in order to get a neutralisation reaction to occur completely. That means if we dilute it by a factor of 100, it would go down to 0.02 mols dm-3 which is close enough to give us accurate results.

Dilution Factor = x100

New Concentration = 2.00 ÷ 100

= 0.02 mols dm-3

Method

Method of Dilution of HCl

Below is the list of all the apparatus and chemicals with amounts needed to do the dilution:

25cm3 of 2.00 mol dm-3 hydrochloric acid
450cm3 of distilled water
25cm3 graduated pipette with pipette pump
250cm3 beaker
250cm3 volumetric flask

Diluting by a factor of 100 in one go is impossible, so I will be diluting it by 10 times twice.

By using the 25cm3 pipette and the pipette pump, collect 25cm3 of HCl with the bottom of the meniscus in level with the fill line on the pipette. This is done to ensure that the correct volume of HCl is obtained so the results will be accurate. After that, empty the pipette into the flask and fill the flask up to its 250cm3 fill line using the distilled water. Again making sure that the bottom of the meniscus is in line with the file line. Now with the stopper on, give the flask a good shake, turning upside down and backup to ensure all of the solution is mixed and the correct concentration is obtained. Empty out the solution from the flask into the beaker. The solution is now diluted down to 0.2 mol dm-3.

Rinse the flask with distilled water to ensure no traces of the solution are left. Then using the pipette again, collect 25cm3 of the diluted HCl solution from the beaker and empty it in the flask. Now again I need to fill the flask upto 250cm3 line by adding distilled water and the with the lid on, shake the flask. Now empty out the beaker and rinse it with distilled water and transfer this newly diluted solution into the beaker. We can now discard all the apparatus apart from the beaker containing the diluted HCl and the 25cm3 graduated pipette with the pump.

This solution is now distilled by a factor of 10 again so in total the HCl is diluted by a factor of 100 giving us our desired concentration of 0.02 mol dm-3.

Method of Titration

Below is the list of all the apparatus and chemicals with amounts needed to do the titration:

25cm3 Graduated Pipette & Pipette pump

50cm3 graduated Burette,
Stand with clamps
Funnel
250cm3 Conical Flask
Beaker containing 250cm3 of 0.02moldm-3 HCl solution.
(Max. of 50cm3 needed for each titration)
Beaker containing 250cm3 of Limewater solution of unknown concentration.
(25cm3 needed for each titration)
Phenolphthalein Indicator
White Tile

First set up the apparatus for the titration. Put the clamps on the clamp stand and using the clamp, slot in the burette. Place a funnel on top of the burette and fill it to max (50 cm3) with the 0.02moldm-3 HCl solution and then remove the funnel. Then using the pipette with the pump, collect 25cm3 of limewater solution (bottom of meniscus in line with the fill line) and empty it in the 250cm3 conical flask. Add 5 drops of Phenolphthalein indicator into the flask containing the limewater solution. This will turn the solution to pink and so when neutralised, it should go colourless. Place the flask underneath the burette (containing the HCl solution) and on top of a white tile.

The first titration (rough): Open the tap gently on the burette so you get the HCl coming down at a slow but steady speed while gently shaking the conical flask. As soon as the limewater solution goes colourless, close the burette tap and record the volume of HCl used for the neutralisation. Now empty out the flask and rinse with distilled water and collect 25cm3 of limewater again using the method above and again put 5 drops of indicator for the next titration.

Further titrations: Open the burette tap so the HCl dropping is at even slower speed than the rough titration. Gently keep shaking the flask and when you are within 1cm3 of the result from the rough titration, close the tap so the HCl is coming down in single drops. Keep shaking until you get the solution colourless again and then record the result.

You need to carry out further titrations until the point where you get 2 results which are within 0.1cm3 of each other.

The results will be recorded to 1d.p as the burette displays it to that accuracy.

After collecting the titration results, I will work out the average titre i.e. the average volume of hydrochloric acid needed to neutralise the 25cm3 of limewater solution. After that, using the balanced chemical equation, I will be able to determine the actual concentration of the limewater solution.

Accuracy of the Equipment:

The 25cm3 pipette has an uncertainty of 0.04cm3 which isn’t that significant so it won’t affect the accuracy of the results significantly.
The 50cm3 Grade B Burette has an uncertainty of 0.08cm3 which again will not affect the results significantly.
The 250cm3 volumetric flask has an uncertainty of 0.02cm3 which is a very minor value and will not affect the accuracy of the results.

Safety Precautions

Must wear safety goggles when near any chemicals.
Hydrochloric acid is toxic and corrosive.
Limewater is Corrosive. Care must be taken when handling it.
Keep the work area safe and tidy; when equipment is no longer required, move it aside or clear it away.
Do avoid spillage; ensure the burette tap is closed before filling and use a funnel.
Remove the funnel from the burette when not in use, this could cause a hazard and any chemicals dripping off the funnel into the burette will cause inaccurate results.
Wash out all equipment before and after use to remove all traces of chemicals which could cause errors in the experiment.

Bibliography

(“Chemistry 1” by B.Ratcliff, H.Eccles, D.Johnson, J.Nicholson & J.Raffan.) I used this textbook to read about the aim of titration and its method. This is a very reliable source as it gives a detailed insight about what titration is, why it’s used and how it’s carried out. This helped me in writing my method of titration properly and make sure it will give reliable and accurate results.
() I used this website to do research on calcium hydroxide and find out what strength base it was. This is also a reliable source as the article goes in depth about the chemical structure of the calcium hydroxide and explains in detail what strength base it is and with scientific justification.
() The page describes how pH changes during various acid-base titrations. I used this website to determine the strength of HCl as a base and looked at the pH curve when strong acid and alkali are added to decide which indicator to use. The information is reliable as it has pH graphs of different strength acid-base titration which shows clearly what happens during the titration and at which pH you would expect the titration to be complete.
() This page describes how simple acid-base indicators work, and how to choose the right one for a particular titration. It had the pH titration graph for strong acid v strong base which is the case in my experiment as I found out through the sources above. The graph helped me decide which indicator would be more suitable with justifications. So I think this is also a reliable and accurate source.
() I used this website to find out information and hazard regarding both reagents HCl and Ca(OH)2. The site gives a detailed student friendly format information about the chemicals with their hazards and safety precautions that should be taken when handling. This site contains reliable and accurate information about the chemicals.